Polyurethane Adhesives Formulation and Additives
Polyurethane adhesives find use across many industries and in daily life. They are capable of bonding to most common materials, including wood, glass, ceramics, metals, and plastics.
This bonding capability is due to significant hydrogen bonding and high polarity.
The versatility of polyurethane adhesives makes them the first choice for the construction and automotive industries.
Low curing temperatures, flexibility, impact resistance, and durability against water are all features that lend themselves well to modern applications. The individual formulations of adhesives give them their unique properties.
Many different additives and raw materials go into the fine tuning of adhesives for specific applications. The development of new formulas for polyurethane adhesives is a competitive field that is full of innovation.
What Are the Basic Raw Materials in Polyurethane Adhesives?
Polyurethane adhesives cure through the reaction of isocyanates with chemicals that contain hydroxyl groups.
The specific isocyanates and hydroxyl-bearing compounds used in the formula can significantly alter the curing speed and the ultimate bond strength.
- Isocyanates are among the most common ingredients in adhesives today. The most prominent are toluene diisocyanates, TDI, and methylene diphenyl isocyanate, MDI. These chemicals contribute to the development of one-component adhesives that do not require a mixture of two separate adhesive components to initiate the curing process. Blocking agents are added to isocyanate formulas to produce adhesives that are activated by heat. Various phenols or other complex alcohols serve as blocking agents for this purpose. Different combinations of individual isocyanates allow for a spectrum of flexibility, adhesion strength, and resistance to particular chemicals.
- Polyols are forms of polyester that terminate in hydroxyls, i.e., polyester alcohol. These compounds are incredibly vicious, making them well suited to formulas that are solely polyurethane-based. More complex polyols tend to exhibit higher viscosities. These variants are more expensive than their low viscosity counterparts, but they also form stronger bonds. Polybutadienes are added to polyol formulas to create hydrophobic adhesives for increased water resistance.
The Two Main Types of Adhesives
Polyurethane adhesives come in both one-component and two-component varieties. One-component adhesives are a single compound that begins to cure after application. Two-component adhesives require two separate components that are mixed, with the mixture then curing.
- The original polyurethane adhesives were simply isocyanates that would react with whatever hydrogen was present on the surface they were bonding to. Today, most single-component formulas are high weight polyisocyanates that rely on moisture in the air to initiate the bonds required for curing. There are also solvent-based single-component adhesives that solidify as their solvent evaporates. Some solvent-based adhesives also cure using residual moisture in the air, with the solvent serving only to facilitate application.
- The cure speed is the most significant reason to choose a two-component adhesive. A one-component adhesive with a higher curing speed is at risk of curing before application. Two-component glue only begins to cure once the two components are mixed, allowing for control over the curing process. Many industrial manufacturing environments use this type of adhesive to ensure that the adhesive assembly step doesn’t become a bottleneck. Applications where moisture from the air is not available also call for the use of two-component adhesives.
What Additives Are Used and Why?
While isocyanates and polyols provide the underlying linking mechanism that creates the adhesion, numerous additives go into any formula. These additives vary widely in their purposes. Some increase strength, some stabilize the formula, while others provide niche properties for niche applications.
- Chain extenders are a class of additive that increases adhesive strength through their reactions with isocyanates. They grow the polymer chain lengths within the mixture. Most chain extenders are diols, typically short-chain such as butanediol or hexanediol. Certain amines are potential chain extenders as well, but find little use due to their reactions being too fast for adhesive use.
- Both one-component and two-component adhesives contain catalysts. They improve adhesive strength but lead to stability issues during storage. Various forms of dibutyltin are the most well-known polyurethane adhesive catalysts. There are also amine and organometallic based catalysts in some formulas.
- Organic solvents are a common component of polyurethane adhesives. Ketones, acetates, and alkene chlorides all find extensive use in the formulation. When a binding application is required, xylene is the most effective solvent. Adhesives with solvents may need flow agents to for practical use. These flow agents include butyrates and vinyl acetate copolymers. Copolymers are especially useful, as most types also improve the range of materials that the adhesive can bond with.
- Many polyurethane adhesive formulas call for the use of fillers. Individual fillers are typically selected for their color. Calcium carbonate, silica, talc, and a variety of clays find use as fillers. Fillers must be thoroughly dry before being added. They are the most common path through which water can enter a formula, leading to the breakdown of certain components and the production of carbon dioxide.
- Adhesion to porous materials is a property that is augmented by the addition of what is called thixotropic materials. Any polyurethane adhesives meant for applications with concrete, paper, or textiles will have one or more of these additives in the formula. The most common additives for this purpose are silica, polyureas, or castor oil derivatives.
- Plasticizers are some of the most varied and complex additives. A plasticizer must be chosen with care as they are prone to producing unpredictable results in combination with other additives. These additives help reduce the hardness and make the adhesive bond’s flexibility. Plasticizers play a crucial role in regulating the thermal properties of adhesives, such as resistance to thermal expansion. Some plasticizers can increase the acceptable ratio of filler in a formula, leading to cost savings.
- Stabilizers improve the UV resistance of the adhesives. They are a common additive in adhesives meant for outdoor applications. Most stabilizers are simple inorganic pigments, like titanium or zinc oxides. Carbon black makes an effective stabilizer against UV degradation if the black color is acceptable for the end-use. There are a variety of organic stabilizers as well, various types of hindered phenols and amines.
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